This invention relates to a heat pump for cooling an automobile. The heat pump is driven by waste heat from the engine. The present invention eliminates the need for a compressor. In conventional air conditioners, a compressor consumes a significant part of the engine horsepower. The present invention also eliminates the need for chlorofluorocarbons, which destroy ozone in the upper atmosphere.
A typical heat load for an automobile air conditioner is 12,000 BTU per hour. It takes about 18,000 BTU per hour to drive the heat pump of the present invention. A typical engine produces about 11 pounds of exhaust gas at 500.degree. C. per horsepower-hour. Thus, if the engine is operating at 20 horsepower, and the gas is cooled by 200.degree.C., the amount of heat produced by the engine, per hour, is EQU (20 hp)(11 lb/hp/hr)(200.degree. C.)(0.45 Btu/lb/.degree.C.),
where 0.45 Btu/lb/.degree.C. is the specific heat of air. The above expression equals 19,800 Btu per hour, which is sufficient to drive the heat pump.
An essential component of the heat pump of the present invention is the hydrogen storage cell described in U.S. Pat. No. 4,599,867. The cell comprises a nest of metal fins that are traversed by tubes that carry a heat transfer fluid. The fins are coated with a metal hydride or with a metal capable of forming a hydride. Also essential to this heat pump is U.S. Pat. No. 4,799,360, entitled "Method of Binding a Metal Hydride to a Surface". The disclosures of the above-cited patents are incorporated by reference herein.
The present invention is an improvement over the apparatus described in my U.S. Pat. No. 4,939,902. In the apparatus shown in the latter application, the hydrides cycle between the temperature at which a cell does the cooling and the temperature at which the cell is regenerated. The problem resides in reducing the temperature of the hydride from the regeneration to the cooling temperature. Whatever part of this cooling is done by evolving hydrogen from the hydride is parasitic cooling that lowers the efficiency of the device. The present invention provides an air conditioner which can reduce the parasitic cooling by as much as a factor of two.